ouijabot

The ouijabot is an omnidirectional ground robot platform designed for general experiments in the plane, including 2D SLAM, collaborative manipulation, and pursuit-evasion. The platform's holonomic dynamics allow it to be well-modeled with a single-integrator model, which make it ideal for a wide variety of planar control and navigation tasks.

The hardware consists of 4 DC motors, which drive omnidirectional wheels, and an embedded Linux computer (Raspberry Pi). This project aims to completely open source all software and hardware. All communication is performed using ROS.

Supported functions

Below are the features or capabilities of the ouijabot which are currently supported.

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Onboard functions:

Velocity control

• Open-loop controller which seeks to track a desired robot velocity (combined linear and angular velocity).

Force control

• Closed-loop controller, which uses onboard current sensors to generate desired wrench (force and torque).

Sensors

• Measures current supplied to each motor by the motor drivers.
• Measures current linear acceleration and angular velocity via onboard IMU.

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Offboard functions:

Velocity teleoperation:

• ROS node which generates and publishes body-frame velocity commands via joystick input.

Force teleoperation:

• ROS node which generates and publishes body-frame force and torque commands via joystick input.

Basic usage

Below we provide general instructions for how to utilize the supported features detailed above. Please see the code for additional details. All nodes typically run in a namespace set by the user's $HOSTNAME variable.

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Velocity control

• Onboard: run roslaunch ouijabot velocity_control.launch to start the velocity control node (velocity_control.py). The node will subscribe to the topic cmd_vel, which uses messages of type geometry_msgs/Twist.

  • Arguments:
    • cmdMode: flag for desired throttle response of ouijabot. If set to "equal", command axes are decoupled; throttle commands in one axis always correspond to the same speed, regardless of other axis commands. If set to "max", then any command with l1-norm >= 1 is treated as "full speed"; at least one motor will run at max RPM. Allows for faster speeds overall of the ouijabot, but with less physical intuition behind commands.

• Offboard: to teleoperate ouijabot, run roslaunch ouijabot_telop vel_telop.launch to launch the velocity command node. The node will read joystick inputs and use them to generate velocity commands, which it publishes to cmd_vel.

  • Arguments:
    • id: index of ouijabot to be commanded. The node will remap the generic cmd_vel topic to /ouijabot$(arg id)/cmd_vel by default. Defaults to 1.
    • velMax_l: scaling parameter for linear velocity commands. Should range between 0 and 1, with 1 corresponding to max throttle available in hardware. Defaults to 1.0.
    • velMax_a: scaling parameter for angular velocity commands. Same behavior as linear velocity scaling; defaults to 1.0.

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Force control

• Onboard: run roslaunch ouijabot force_control.launch to launch the force control node (force_control.py). The node will subscribe to the topic cmd_wrench which uses messages of type geometry_msgs/Twist. It uses onboard current sensors to run a closed-loop PID controller to generate the desired wrench.

  Note: params.launch must be run before force_control.launch; the PID parameters are set as ROS parameters in this file. This is typically done once, offboard, as part of robot setup. These parameters may easily be tuned by setting the ROS parameters manually.

• Offboard: run roslaunch ouijabot params.launch, once, to set force controller parameters. Then run roslaunch ouijabot_telop force_telop.launch to teleoperate the ouijabot in force control mode. The node will read joystick inputs and use them to generate wrench commands, which it publishes to cmd_wrench.

  • Arguments:
    • id: index of ouijabot to be commanded. The node will remap the generic cmd_vel topic to /ouijabot$(arg id)/cmd_vel by default. Defaults to 1.
    • force_scale: scaling parameter for max force command, in Newtons. Defaults to 100.0.
    • torque_scale: scaling parameter for max torque command, in Newton-meters. Defaults to 5.0.

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Sensor readings

• Current sensors: run roslaunch ouijabot current_read.launch to run the current sensor node current_read.py. The node reads the current sensors on the motor controllers and publishes the values to the topic current as a vector of floats.

  Note: *Do not run current_read.py at the same time as force_control.py; the force control node reads/publishes the current sensors as part of its control loop and the conflicting reads could cause timing errors.

• IMU: run roslaunch ouijabot imu_read.launch to run the IMU node imu_read.py. The node will read measurements from the IMU over I2C and publish them to the topic imu as messages of type sensor_msgs/Imu.